MRI apparatuses are widely spread in medical facilities similarly to diagnostic apparatuses using X-rays. The reason is that the MRI apparatus has a different function from the diagnostic apparatus using X-rays. Not only the diagnostic apparatus using X-rays but also the MRI apparatus is an indispensable diagnostic apparatus for more reliable examination in diagnosis. The diagnostic function of the MRI apparatus is to obtain not only the morphological image information but also the functional diagnostic information. For example, the MRI apparatus can clearly draw a cerebral infarction lesion in the early stage of the disease. In particular, in an MRI apparatus using a magnet with high magnetic field performance in which the magnetic field strength exceeds 1 tesla and the magnetic field homogeneity is 3 ppm or less, many new diagnostic functions have been developed taking advantage of the magnetic field performance so that actual clinical applications are made.
In order to achieve the high diagnostic function of the MRI apparatus, it is necessary to increase the magnetic field strength of the imaging space where a subject is disposed and to increase the magnetic field homogeneity. The increase in the magnetic field strength increases the strength of a nuclear magnetic resonance (NMR) signal detected from an examination area. Accordingly, since a signal-to-noise ratio or a contrast-to-noise ratio of a diagnostic image is increased, the quality of the diagnostic image is improved. In addition, the increase in the signal-to-noise ratio indicates that a high-speed imaging technique for shortening an examination time becomes possible. On the other hand, since even a slight difference in resonance frequencies of NMR signals acquired from the examination area can be identified by increasing the magnetic field homogeneity, the spatial resolution of the diagnostic image or the analytical capability of spectral diagnosis can be improved.
In order to acquire a high-quality examination result in a diagnostic image or the like, not only the performance of the MRI apparatus but also realizing a comfortable examination environment, which relieves the tension of the subject, is important. In order to improve the examination environment, an open MRI apparatus with an open-structure imaging space where a subject is disposed has been realized (PTL 1). In the MRI apparatus of PTL 1, an open examination space is realized by using a superconducting magnet in which superconducting coils are disposed above and below the space, in which a subject is disposed, interposed therebetween.
On the other hand, in a superconducting coil formed by pouring thermosetting resin (epoxy resin) between wound superconducting wires and curing it, distortion by cooling occurs due to a difference between the thermal contraction rates of the superconducting wires and the resin, as disclosed in PTL 2. If the superconducting wires move by several micrometers due to this distortion, the superconducting wires are locally heated. As a result, a quench by which a change to the normal conducting state is made occurs, as is known well. If a quench occurs in the superconducting magnet of the MRI apparatus, not only does the superconducting magnet not function as a superconducting magnet any more, but also a large amount of refrigerant is vaporized by the heating and is emitted through en emergency exhaust port. For this reason, it is necessary to avoid the occurrence of a quench in the MRI apparatus installed in the hospital and the like.
In order to avoid the sudden occurrence of a quench in an MRI apparatus which holds a persistent current over a long period of time, PTL 2 proposes reducing distortion energy generated in superconducting wires and resin by repeating the electromagnetic force generated at the time of excitation and demagnetization several times (at least 3 times) or an adjustment method of a superconducting magnet which releases distortion energy of a superconducting coil by causing a quench forcibly with the heat from the outside.